1. Field of the Invention
The invention relates to an injection unit for an injection-molding machine for processing plastics or other plasticized materials.
2. Description of the Related Art
Injection-molding machines for processing thermoplastic materials, for example, have been known for many years. In such an injection-molding machine, usually plastics pellets are heated up and melted in a heated plasticizing cylinder, in which a screw rotates, and the melt is subsequently injected under high pressure into the cavity of an injection mold. The molten plastics material is then cooled down, so that it solidifies and is given its final shape. The cooled-down injection-molded parts are ejected after the mold has been opened. Apart from its injection unit, an injection-molding machine also includes a closing unit for opening and closing the split injection mold. Apart from a screw cylinder, the injection unit also includes an injection nozzle for injecting the molten material into the closed mold. Arranged in the screw cylinder is a screw, which can be moved back and forth in the axial direction to execute the injection function, the length of the axial path of displacement directly determining the amount of material injected.
An electric motor is usually used for the rotational drive of the screw, with a mechanical gear mechanism often being interposed. For the axial movement of the screw during injection, hydraulic drives are mostly used.
In particular for producing relatively small injection-molded parts, injection-molding machines which have a purely electrical drive have become known in recent years. That is to say that the axial longitudinal movement of the screw is also performed by means of an electric drive.
EP 0 723 848 B1, which corresponds to U.S. Pat. No. 5.679,384. discloses a number of different embodiments of such fully electrical injection-molding machines. In one generic type of injection-molding machine, it is provided that the screw can be moved together with the electrical rotational drive unit, which is coupled to the screw, on a carriage, the moving of the carriage taking place by means of an electromotively driven spindle drive. Consequently, in this case two electrical rotational drives are used, one of which uses the spindle drive to convert the rotational movement into a linear movement for the injecting operation.
Another type of injection-molding machine described in EP 0 723 848 B1 provides two electrical hollow-shaft motors lying one behind the other, the first of which has in its rotor a spindle nut for a ballscrew, which interacts with a ballscrew connected directly to the screw of the injection-molding machine, while the second electric motor has on its rotor a splined-shaft nut, which interacts with a splined shaft which directly adjoins the ballscrew. The length of the splined shaft corresponds at least to the axial path of displacement of the screw for the injecting operation. During the return movement of the screw in the material-collecting phase of the injection-molding cycle, both motors are driven at the same speed and in the same direction of rotation. As a result of the counter-pressure formed by the injection nozzle, in this case the screw is pushed linearly backward. Since the splined shaft is in engagement with the splined-shaft nut, in this case the rotational drive remains undiminished. During the injection phase, the motor for the rotational drive is switched off and only the motor with the spindle nut continues to move. Since the screw can no longer rotate, the rotation of the spindle nut enforces an axial movement of the screw in the direction of the injection nozzle and the melt material which has collected in front of the screw is injected into the injection mold.
Injection-molding machines in which the rotational drive of the screw is generated by means of an electrical rotating motor and the linear displacement of the screw for the injecting operation is generated by an electrical linear drive have also already become known. In this case, four linear motors in a parallel arrangement are used, the linear motors being connected to one another to form a cuboidal, housing-like group. The electrical rotational drive and the linear motors are in this case arranged one behind the other in the axial direction. The advantage of the electrical linear drive lies in particular in the high accelerating capability and, as a result, great injection speed. The comparatively great overall axial length of such an injection unit can be seen as a disadvantage.